1. Field of the Invention
The present invention relates to a method of fabricating photovoltaic solar cells equipped with surface collector grids. More particularly, the invention relates to a method of coating the surface of a silicon substrate with a metal collector grid in a manner which obscures only a minimum of the surface while at the same time providing good conduction of carriers to the external circuitry of the silicon solar cell.
2. Description of the Prior Art
In the past photovoltaic devices such as solar cells have been prepared by depositing collector grids on the surface of the photovoltaic substrate of the cell by vacuum evaporation or sputtering of a metal or conductive substance. The deposition step is frequently followed by a step in which the substrate is further plated with a conductor to further increase the current conducting capacity of the collector system. No matter how the collector system is fabricated, the function of the collector is to provide nearby available conductors for the carriers which are generated by the absorption of photons of light in the semiconductor substrate. In the past the impurity concentration or doping level of the wafer from which a photovoltaic silicon solar cell is fabricated, has been substantially limited to a level such as about 10 ohm-centimeter resistivity. If the level of doping is increased such that a lower resistivity material is formed, the conversion efficiency of the device is reduced because of recombination losses of electrons which are unable to reach the widely-spaced collector grid wires. One method of overcoming this problem has been to provide a grid collector system containing more numerous collectors which are more closely spaced. However, it would be more desirable to be able to employ solar cells which are more heavily doped such that they possess a resistivity on the order of 1.0 or 0.1 ohm-cm. A type of collector system which would be ideal is a grid-like surface collector which would obscure a minimum area of the surface of the substrate exposed to light while providing a low-resistance current path for the carriers. The lifetime characteristics of the carriers is an important property to consider in the preparation of a collector system so that the carriers will be able to reach the collector wires of the system and flow through the external circuitry of the photovoltaic system to perform work. One of the most significant deficiencies of the prior art collector systems is that many of the carriers are lost before they can be collected, even in the best of the current collector designs. Collector systems are generally designed with the main conductor or channels being of relatively large current conducting capacity with many more smaller current conductors attached to the main conductive channels.
Methods are known in the art for providing photovoltaic substrates with a transparent or at least translucent layer which functions as one of the electrodes necessary for the passage of the current generated by incident photons on the device. Thus, U.S. Pat. No. 3,811,953 shows a photovoltaic device coated with a conductive, transparent layer of Cd.sub.2 SnO.sub.4 which functions as a conductive electrode. U.S. Pat. No. 2,766,144 shows a photovoltaic device coated with a translucent coating of a Group III element such as indium or Group V elements such as antimony as a conductive electrode. U.S. Pat. No. 2,870,338 shows a transparent conductive layer on a photovoltaic substrate of a material such as tin oxide. However, these devices have the disadvantage that such transparent, conductive surfaces as Cd.sub.2 SnO.sub.4 and SnO.sub.2 lack highly conductive channels within or on the conductive layer which adversely affects the ability of the device to effectively transport the current generated by the incident photons to the external circuitry of the device. Moreover, photovoltaic substrates which are coated with a layer of a Group III or V element have the disadvantage that the current generating ability of the device is diminished because of the reduced transparency of the layer.
The present invention meets the need for an improved collector system for photovoltaic devices in that it has the advantages of the high conductivity of a metallic mesh, it is designed to obscure only a minimum area of the surface of the photovoltaic device exposed to light with opaque material while providing good conductivity for the carriers generated by light photons to the external circuitry of the device and it maintains the advantage provided by a transparent electrically conductive coating in contact with the total active diffused surface of the photovoltaic substrate.